Saturday, May 16, 2020

Cratering - Marking the 40th Anniversary of the Mt. St. Helens Eruption

Photomural image by Richard Gordon Bowen, May 18, 1980. Courtesy of the Bowen Family.
May 18, 1980. If most people remember this date at all, they will recall it was the sunny morning that Mt. St. Helen's in southwest Washington's Cascade Mountains let loose with a powerful blast that tragically killed 57 people. The 40th anniversary of that eruption is this Monday, May 18. I found out about the blast upon ending a river trip in the Grand Canyon. But first...

... The Portland Art Museum is sponsoring an exhibit to commemorate this historic geologic event. I learned of the exhibit, now viewable virtually and online, through my friend Dixie Watkins and his wife Elaine. They sent me a link that will take you on this virtual tour, which is outstanding! This is the preamble to the exhibit and tour:

"The Portland Art Museum proudly presents this tribute to Mount St. Helens on the fortieth anniversary of the eruptions of 1980. Spanning the period from 1845 to the present, this exhibition is the first survey of works of art inspired by the mountain. Although 175 years is barely a blip in geologic time, the art bears witness to an extraordinary era in the long, cyclical life of the volcano. The beauty of Mount St. Helens has ranged from bucolic to savage. Before the eruptions, painters delighted in depicting its pleasing conical shape rising high above the verdant landscape. The 1980 eruptions challenged artists to capture the thrilling and terrifying displays of nature’s sublime power. When the smoke cleared, the new apocalyptic face of Mount St. Helens compelled the depiction of its haunting majesty. Since then, the rapid return of life to the mountain has captured the attention of photographers as well as scientists from many fields. Although the volcano seems to have reclaimed its serenity, some artists have begun to look to the future. Mount St. Helens will erupt again. We are pleased to welcome you to this celebration of a great wonder on our horizon."

*** You can view the virtual exhibit at this link:***
Paul Kane, 1810–1871) Mount St. Helens, 1849–1856. Oil on canvas. Royal Ontario Museum, Toronto
I remember the day 40 years ago. I was rowing for the very first time a raft down the Grand Canyon. It was a US Geological Survey research trip with veteran geologist Don Elston leading the way. Don was studying remnant paleo-magnetism recorded in some of Grand Canyon's rocks and he conducted river trips to access the sites. Before this trip began, he was short one boatman - and in the final moments before the trip began.

Don had open heart surgery just six months before this trip, which was to be his first since his operation. He was excited and perhaps just a bit anxious about the trip. His wife Shirley and nephew Charlie were on the trip as well.
Gary Mercado, a fellow geology student with me at Northern Arizona University, had previously rowed boats for Don in these studies. But just before the trip began, Gary received word that he got a job with the USGS in Denver. We had both just recently graduated with geology degrees the week before the trip began but now Gary needed to decline his previous commitment to row this trip. Luckily for me, he asked if I was interested in being his replacement. I jumped at the opportunity, not giving it a whole lot of forethought. In those days, I said yes to anything that sounded good, oftentimes not thinking it through (like the one and only time I jumped out of an airplane in 1976).
Gary put me in touch with Don Elston and before I knew it I was rowing a boat down the Grand Canyon!

Up to this point I had only been a passenger on the river in Grand Canyon and that was just half of a trip from Phantom Ranch on down. Like everyone else back then, you learned by doing - just get in the row seat and go. Do what I do. So after the trip started and we would scout a rapid, people would ask me what that particular rapid in Marble Canyon looked like. Not wanting to appear inexperienced, I quickly came up with the honest, but incomplete answer, that, "I've never seen this rapid at this level of water." It seemed to work. (Although to this day I'm sure many of those intelligent folks figured out that I was the new guy. I was so scared from the get go and I made too many mistakes. But I was thrilled for the opportunity to row a boat and there were so many big name geologists that were along on the trip. I remember one - Max Crittenden, one of the first geologists to ever describe metamorphic core complexes. There were others whose names I have now forgotten.

Many interesting events happened on this trip. This was the year that Glen Canyon Dam filled the Powell reservoir for the first time so there was 30,000 cfs coming down the river. It was HUGE! I was rowing a very large 22' snout boat with 12-foot oars. As skinny as I was in those days, and with no prior rowing experience, I couldn't really control the boat. One day, camp was to be made at the Clear Creek beach but I missed the pull in. Don wanted to walk up to the horizontal waterfall and look at some rocks there. It was late in the day we were to camp there. But because I missed the pull-in, we had to continue down to Phantom Ranch.

There was no way to camp there for river parties but we did take time walk up to the Ranch so Don could make a phone call. While we were there, I encountered my old Park Service boss, who was giving an orientation tour of the Ranch to the brand new Superintendent of the Park, Dick Marks. I was not on good terms with my old boss and as the formalities were exchanged between the various members of our group, I eyed my ex-boss warily, as he did me.

We left Phantom but it was too late to go vary far, so we had to find a camp fast. We stopped just a few hundred meters downstream on a cobble beach just across from the River Trail. It was illegal for river parties to camp there in those days but we really had no other choice. It was getting dark and Horn Creek Rapid loomed ahead just two miles. While we setting up camp, wouldn't you know it, the two rangers we saw at Phantom Ranch were hiking up to Indian Garden on the River Trail and stopped across the way to look at our camp. They knew who we were. When the trip was over, Don received a citation in the mail from Grand Canyon National Park for illegal camping. For years afterwards, Don always joked that the citation was labeled "Department of the Interior vs. Department of the Interior".

A few days later, I got into an eddy I could not row out of. It was just above Tapeats Creek. Round and round we went for at least 20 minutes and I became thoroughly exhausted. Finally, one of the younger geologists in my boat stepped in next to me and we each took one oar and finally made it out. Man that was big water!

Albert Bierstadt (American, born Germany, 1830–1902). Mount St. Helens, Columbia River, Oregon, 1889. 
Oil on canvas. Collection of L.D. "Brink" Brinkman, LDB Corp, Kerrville, TX

On May 18, 1980, the day that Mt. St. Helen's blew its top, we were blissfully unaware of it and getting ready for our run of Lava Falls Rapid, the biggest, meanest rapid in the whole canyon. To say I was scared to run this monster for the first time would not convey in a proper way how I felt. I was terrified. And apparently, Don had a history with this rapid that caused his wife Shirley to begin to fret as much as me. While we scouted the rapid, it was determined that Shirley would ride with me on the biggest boat, and Don's nephew,Charlie would ride in the smaller boat with him. Remember, this was just six months after Don's open heart surgery.

Finally, I got in my boat. I was hyperventilating as I pulled away from shore and entered the tongue of the rapid. I guess I was lucky to have such a large craft but one still needs to bee spot-ion for the entry through the tongue. And myriad swirling currents play havoc on smaller boats in there. I remember very little about the specific run I made. Someone on shore took a movie picture of it and I have a copy of that somewhere. But it must have been a good run because I don't remember it! At the foot of the rapid I had a hard time stopping but eventually pulled over on the left bank below Prospect Spring. We couldn't see the rapid from there but waited for the other boats in our party to come down.

As we bobbed in the eddy below the spring, time seemed to drag on forever. Where were they? Before long, Shirley started to worry that no one was coming down. "Where is Don," she repeated. I tried to comfort her as best as I could. But I started to worry as well. Shirley hadn't wanted Don to row this trip so soon after his surgery and wanted to be on the trip with him. She worried audibly that something bad had happened. I tried to calm her down.

Meanwhile, up north, Mt. St. Helens was roaring with its gigantic blast. We would not find out until the trip was over what happened, although we were all talking about it on the trip, as it had been rumbling for months. All geologists back then were excited about what might happen with the volcano.

Finally, we saw something coming down the river. It was an ice chest bobbing in the water. Uh-oh. Something else came along just after that - a life jacket! Oh no, that's not good. Shirley started to cry. I immediately pulled out into the current of the river to be of assistance anyway I could. Just by chance at that time Don came by my boat bobbing in the water. Charlie was very close to him trying to reach him in case he needed help. I pulled them both into my boat as we began to go through Son of Lava Rapid. Someone in another boat wrangled his boat ashore. He had misplaced his position in the tongue of the rapid, the swirling currents grabbed his keel and he ran right over the ledge hole! No man's land!

Forever afterwards, Shirley Elston thought of me as the one who saved her husbands' life. For the rest of our friendship, spanning many decades, I could do no wrong. She and Don took that day at Lava Falls all the way to their graves and I was honored to be invited to Don's memorial. I remain forever grateful to them both in giving me the chance to be a boatman in Grand Canyon.

Look at the online exhibit and remember Mt. St. Helens 40 years on!

Frank Gohlke (American, born 1942). Aerial view: Landslide—debris flow area—looking east toward Spirit Lake, 
5 miles north of Mount St. Helens, 1982; printed 2005. Gelatin silver print. Courtesy of Artist and Gallery Luisotti

Thursday, May 07, 2020

More Cratering in Northern Arizona - Rattlesnake Maar

Photo by Dawn Kish - From the top of the Rattlesnake scoria cone; the reddish arc behind the figure is Rattlesnake maar
An unusual and little-known landform on the eastern side of the San Francisco Volcanic Field is the Rattlesnake maar near Winona Arizona. As the Covid-19 pandemic set in, some friends, my wife and I decided to visit nearby volcanic craters — to get exercise, breathe fresh air, and hone our geologic skills. (Admittedly, I was the only geologist on these excursions and none of the others wished to hone anything of the kind. Yet they at least admitted to wanting to know more)! When I was a graduate student at Northern Arizona University in the 1980s, I took a class called Volcanology and the lab consisted of visiting a different volcano every week! It was the best lab class I ever had. Rattlesnake maar was one of the features we studied.

View of the west side of the maar where the truck is parked. We are on the rim of the maar at this location and simply walked up the gentle slope on the two-track road around to its crest. The snow-capped San Francisco stratovolcano lies in the distance to the west, with other cinder cones in between. Cinder is a term in common usage but geologists also use terms such as lapilli and scoria. All are any pea-sized ejecta that forms when droplets of molten lava are thrown into the air and cool and harden. Scoria contains more vesicles, being the basaltic equivalent of pumice. Thus, the cones can properly be referred to as cinder cones, lapilli cones, or scoria cones.

On top of the Rattlesnake maar looking west. Note the dip of the tuff to the north (right) that was thrown out when the maar erupted (see my detailed description of maar eruptions below).

Looking south to the scoria cone that formed after the tuff cone. The evidence for this is that the southern part of the tuff ring is buried beneath the scoria cone. This was a great hike. For the geologist, I include the next section on maar volcanoes.

Maar's are broad, low-relief volcanic craters found not uncommonly on Earth's surface. Check out this Wikipedia site for some examples of maar's found around the world. They form when hot magma rises into the upper crust and interacts with water. On continents this usually is groundwater but can also be lakes or rivers. When the magma encounters the water a phreatomagmatic eruption occurs. These are violent steam ejections that emplace tuff around the vent. Krakatoa in Indonesia, which erupted in 1883, is an example of an especially large and explosive phreatomagmatic eruption.

Geologists are also interested in the subsurface structure of maar's. The diagram above is used courtesy of Dr. Steve Semken at Arizona State University. It shows a typical carrot-shaped diatreme that underlies a maar volcano. One of the most famous exposed diatremes is Shiprock on the Navajo Nation in New Mexico.

Dr. Semken created this cross-section that places two different, but nearby surface features one on top of the other. They share a common ancestry but one (Shiprock) is the more deeply eroded feature. In other words, before Shiprock was so deeply eroded, it was covered by something akin to the Narbona Pass crater.

Dr. Semken, along with colleague Larry Crumpler of the New Mexico Museum of Natural History, also created this photo collage of the two features. On the bottom is an aerial photo of Shiprock with its presumed diatreme margins added. These project upwards into a second aerial photo of the Narbona Pass crater. I think this image really relates two different parts of maar formation.

This is an excerpt of the USGS geologic map - MF-1960. I have colored some of the pertinent features using the Macintosh Keynote program. The tuff ring of Rattlesnake maar is colored dark purple. Note how it is not present on the south. The Rattlesnake scoria cone is colored lighter purple and overlaps the presumed southern portion of Rattlesnake maar. The brown color denotes the lava flows that issued from the scoria cone and that flowed down the regional slope. Another scoria cone and associated lava flow (not colored), Merrill Crater, can be seen to the east of the Rattlesnake cone.

The map is called "Geologic map of the east part of the San Francisco Volcanic Field, north-central Arizona." You can access the USGS map here:  MF-1960

More cones to come! Thanks as always for reading.